A medical ultrasound system forms an image by acquiring individual ultrasound lines (or beams). The lines are adjacent to each other and cover the target area to be imaged. Each line is formed by transmitting an ultrasonic pulse in a particular spatial direction and receiving the reflected echoes from that direction. The spatial characteristics of the transmitted wave and the characteristics of the receive sensitivity determine the quality of the ultrasound image. It is desirable that the ultrasound line gathers target information only from the intended direction and ignores targets at other directions.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused in a selected zone along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal zone of each beam being shifted relative to the focal zone of the previous beam. In the case of a phased array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal zone can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal zone in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element. The time delays are adjusted with increasing depth of the returned signal to provide dynamic focusing on receive.
In a typical ultrasound system the beamformer control is a significant contributor to the performance and cost of the system. Dynamic receive delay and apodization control must be generated for each beam and channel at a high rate. Often as many as 512 beam channels are required for a high-end system with an update rate as high as 10 MHz. Calculating delays and apodization for an ultrasound beamformer during dynamic reception requires complex calculations including transcendental functions. Normally image quality is traded off for cost by using second or third-order approximations for these functions. Additionally large memories are frequently used to store precalculated controls for predetermined beam positions and parameters. Using stored precalculated values limits the ability of the system to optimally readjust beam position or parameters dependent on scanning situations. Thus image quality is compromised to achieve faster control response times.
There are a wide variety of beamformer control designs currently in use. All use some combination of large parameter random access memories (RAMs), state machines, complex calculations and approximations. Thus they fall short in one or more areas: agility, cost or precision.
Agility means the ability to change the beamforming setup for an entire imaging configuration in a time much less than 1 sec, where the setup includes vector phase center (aperture) positions, steering angles, f-numbers, and focal positions, and the imaging configuration includes all vectors (beams) displayed on the imaging console. Agility also means the ability to begin receive delay computation in mid-flight, i.e., immediately before a deep region of interest.
The controllers described in many patents use state machines. Unfortunately these state machines must begin running immediately after transmit. They cannot easily be reloaded for a different receive focus significantly after transmission. For some high-frame-rate applications, particularly for volumetric imaging, it is desirable to start reception just before echoes from a deep region of interest are received. In addition, at least one prior art method uses parameters which are beam and element dependent. This results in an extremely large number of parameters, involving functions such as square root and sine. This makes agility impossible.
Other systems include inherent approximation, which limits the precision of the beamforming time delays. This results in a loss in image quality, most often manifested by poor contrast resolution, i.e., poor ability to detect very subtle differences in tissue-reflected intensities, or in cystic clearing.
More accurate systems use complicated logic to perform the required calculations. Since these circuits must be duplicated according to the number of channels, they can become a significant portion of the cost of high-channel-count systems.
There is a need for a new beamforming architecture which does not require large memories for storing precalculated beamforming values, which works equally well with transducer arrays of any geometry, and which is agile, precise and low in cost.